libc: Add stubs.versions=R

Add version R to libc so that APEXes can rely on version R stubs of
libc.

Bug: 152655956
Test: vendor/google/build/build_mainline_modules.sh
Change-Id: I70d17702d9885760af658e23ac6e3106405ff3bc
diff --git a/libc/Android.bp b/libc/Android.bp
index 51db9b7..2b1c7ca 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1698,6 +1698,7 @@
         symbol_file: "libc.map.txt",
         versions: [
             "29",
+            "R",
             "10000",
         ],
     },